1. Field of the Invention
The present invention relates to a moldable polyphenylenesulfide which is used for an injection molding or an extrusion molding. More particularly, it relates to a moldable polyphenylenesulfide from which a molded product having excellent mechanical properties and electrical properties can be obtained though an extrusion molding of a polyphenylenesulfide is considered to be difficult.
2. Description of the Prior Arts
Polyphenylenesulfides are polymers having units ##STR1## which are usually referred to as PPS. In the specification, the polyphenylenesulfide is referred to as PPS.
PPSs have characteristics for both of a thermoplastic resin and a thermosettable resin and can be moldable by heat-melting as a thermoplastic resin and also crosslinkable as a thermosettable resin such as a curable paint.
PPS molded products have excellent chemical resistance and excellent mechanical property in wide range of a temperature and excellent hardness at high temperature which are required for engineering plastics.
PPSs have been produced, in an industrial scale by Phillips Petroleum Co. Ltd. and commercially available as "RYTON" (trade mark).
It has been known to produce PPSs by various processes. The commercially available PPSs may be produced by reacting p-dichlorobenzene with sodium disulfide in a polar solvent. The commercially available moldable PPSs include V-1 grade (noncrosslinked); P-2, P-3 and P-4 grades (crosslinked; crosslinking degree P-2&lt;P-3&lt;P-4) and R-6 grade (pellet crosslinked) (V-1, P-2, P-3 and P-4: powder) and R-4 grade (glass fiber reinforced pellet). Compositions incorporating a filler, a lubricant or the other additive are also commercially available.
As usages, V-1 is used for a slurry-spray coating and immersing coating; P-2 is used for a fluidized bed coating; P-3 is used for a powder coating; P-4 is used for a compression molding; R-6 is used for an injection molding. The usages are not critical.
The crosslinking reaction of PPS is carried out by heating noncrosslinked PPS in the presence of oxygen usually in air at a temperature of lower than 288.degree. C. as a melting point of the noncrosslinked PPS, preferably at about 250.degree. C. A chain extension is caused together with the crosslinking by the heating, whereby the melt viscosity of PPS substantially increases until it forms an insoluble and non-meltable resin. Usually, at the desired crosslinking degree, PPS is cooled and a moldable powder or pellet is obtained.
The inventors have found various troubles caused by a generation of a gas from PPS in a step of preparating various method products by an injection molding of moldable PPS. The reason is not clear, but it is presumed that the main reason is caused by evaporation and decomposition of low molecular weight PPS from the fact that the low molecular weight PPS is found in the polymer as a byproduct in the polymerization and low molecular weight PPS is formed by partial cleavage of PPS chain in the crosslinking step. The components of the gas are mainly organic materials and small amounts of sulfur compounds such as SO.sub.2. The gas is remained in the PPS molded product to deteriorate its mechanical characteristics and electrical characteristics. Moreover, the gas may cause corrosion of a metal mold or an inserted metal.
On the other hand, there is the other disadvantage that molten viscosity is remarkably increased by melting PPS in an injection molding machine.
Since the variation of the molten viscosity is large, it is difficult to maintain the conditions of the injection molding (injection pressure and injection rate) whereby a stable molding cannot be attained.
On the other hand, it has been considered that an extrusion molding of PPS cannot be practically attained.
There is no extrusion molding grade in the commercially available PPSs. The main reason that PPS could not be used for an extrusion molding, is caused by severe variation of molten viscosity of PPS in the processing of PPS. That is, when a partially crosslinked PPS which is commercially available is melt-extruded in an extrusion molding machine, the viscosity of the molten PPS remarkably increased and the degree of the increase of the molten viscosity is not constant but is irregular, whereby it is impossible to extrude a constant amount of the molten PPS. When the molten viscosity is remarkably increased, a rate of the extruded PPS under a constant extrusion pressure is remarkably reduced or sometimes, the extrusion is not performed. It is impossible to apply a method of increasing the extrusion pressure depending upon increasing the molten viscosity because the rate of the increase of the molten viscosity is remarkably high. Accordingly, even though PPS can be extruded by an extrusion molding machine, it has been difficult to obtain a molded product having a desired shape because of variation of the extruding rate. For example, when a sheet is formed by the extrusion molding, the thickness of the sheet is remarkably varied. Sometimes, the sheet is cut by decreasing the extruding rate to zero.
The second reason why the extrusion molding cannot be carried out is remarkably low molten viscosity of noncrosslinked PPS. The molten viscosity of PPS can be increased by increasing crosslinking degree. However, the variation of the molten viscosity of PPS is remarkably increased by increasing the crosslinking degree. It has not been known to obtain PPS having relatively high molten viscosity but low variation of molten viscosity.
The third reason, is to generate the gas in the step of melting PPS. The gas generation causes deterioration of the mechanical characteristics and electrical characteristics as well as those of the injection molding. Moreover, the gas generation may cause corrosion of a metal mold. When a film or a sheet is prepared by the extrusion molding of PPS, there are troubles of formation of voids on its surface or formation of holes. These troubles should be dissolved to attain an extrusion molding of PPS.
Among these troubles, the gas generation and the severe molten viscosity variation are common troubles in both of the injection molding and the extrusion molding. The low molten viscosity is the inherent trouble in the extrusion molding.
If PPS having desired molten viscosity can be obtained, the latter trouble can be overcome. That is, PPS having low molten viscosity can be used for the injection molding whereas PPS having high molten viscosity can be used for the extrusion molding.